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\begin{abstract}

ARQ protocols are commonly used in underwater acoustic communication channels.
These channels are half duplex in
nature and predominantly stop-and-wait ARQ protocol or its variants are used
for reliable data transfer.
Also, the propagation delay is much longer in these channels compared to the terrestrial wireless networks. 
Considering the large propagation delay and stop-and-wait ARQ variant protocols, 
the throughput decreases because the transmitter remains idle
for one round-trip time. Although the Forward Error correction (FEC) technique 
can be used to correct the errors but it is not practical to find a FEC which could 
correct all possible errors.

As the propagation delay between any pair of nodes in an underwater acoustic
communication link is high, it is possible for both the nodes to
transmit packets which cross each other in the medium and still be able to
receive correctly ( as half duplex node switches from transmission mode to receive
mode at the time of receiving and vice-verse). This is based on a simple
juggling phenomena where objects are juggled between two hands. At any point in 
time a hand will be either receiving or throwing(human hand works as a half
duplexer).

Similar
 to the juggling concept, underwater communication channel can also be configured  where two
  communicating nodes have multiple packets in medium in both directions even though they cant 
  receive and transmit at the same time . Combining this with ARQ will
  increase throughput and reduce the overall propagation delay.

\end{abstract} 
